The images on this page illustrate
our unique rotary shear apparatus
used in friction studies. The advantage of the rotary shear geometry is that the
ring-shaped samples can be rotated around as many times as desired and so
arbitrarily large slip can be attained at elevated confiing pressure. Both photographs and drawings are on
this page to adequately show the machine. The labeled drawings show some of the functions of the various parts. Comparison with the
photographs allow one to see what they really look like.

The apparatus is
interfaced to a Hewlett Packard UNIX workstation and Hewlett Packard 3852 Data
Acquisition and Control boxes. Experiments are conducted by entering commands
into the computer, either manually or by a predetermined set of instructions in
a "procedure file." Output is to a display screen, a digital plotter
for real-time display of the history of the experiment, and to disk for later
analysis. Some of the photographs show this equipment.

The middle part of the rotary shear apparatus, showing the components that
produce the rotary motion. The small schematic diagram of the machine
shown just below to the left, and the larger version of the diagram below that,
show and label these features. The electro-hydraulic motor that provides the
primary rotary motion is shown projecting toward the viewer at the bottom of the
photo. The middle platen is just above the level of my waist, and the main
thrust bearing rests on it. The rotary servo is at the level of my shoulder. The bottom of the pressure vessel is shown projecting down from the
middle platen, and the piston with a slip-ring assembly for carrying the signals
form the internal load and torque cells is seen just below that. The 1.0 GPa gas
pressure intensifier and 1.0 GPa remotely operated valves are show in right
part of the photo near the bottom and top respectively.

A three-part summary diagram that shows with pink dotted lines the relations
between the overall machine, the sample assembly with internal displacement
transducers, and the sample with its sliding jacket assembly. Enlargements of
each of these three diagrams are below.

Diagram of the overall machine on the left and a photo of it on the right
(prior to construction of a wooden platform that allows access to the top
end).

The load, torque, rotary displacement, and axial displacement are
all measured by transducers inside the pressure vessel, the load cell, the
torque cell, the resolver, and the LVDT, respectively. Therefore, no correction
needs to be made for friction at the pressure seals. Also, as shown in more
detail in the diagram of the sample assembly below, nearly all of the
elastic distortion of the machine is removed from the displacement measurements.

Both
the axial and the rotary motion can occur under high speed servo control. In the
case of the axial motion, this allows constant normal stress if the feedback
signal for the servo is the axial force on the sample, measured with the
internal load cell. In the case of rotary motion, the servo allows the stiffness
of the machine to be electronically increased by using the resolver to provide
the feedback signal.

More
detailed photos of the machine. The wooden platform is just above my head in the
left photo. The pore pressure system above the platform mostly obscures viewing
the large pressure vessel. The right photo shows details of the gas pumping
system, and the axial and rotary drive system. The spiral arrangement inside the
top of the tie bars acts to guide hydraulic hoses that drive the high speed
rotary servo system as they wrap around the machine as displacement occurs. When
the rotary servo system is used only three revolutions can be made because of
limited space for the hoses to wrap around. If the hoses are disconnected, as in
the first photo on this page, there is no limit to the amount of rotation
available, but rotary servo cannot be used.

A cross-section of the sample assembly shows 1) the resolver
that measures the rotation of the bottom sample relative to the top
sample, 2) the LVDT that measures the axial motion between the two sample
halves, and 3) the alignment bearing.

The resolver body and the LVDT body are mounted on a tube that is
attached near the top sample, and the inner rotating shaft of the resolver
and moving core of the LVDT are mounted via the wedge assemblies near the
bottom sample. The bellows allows accurate transmission of rotary motion
to the resolver, while preventing axial motion of its rotating shaft that
would ruin its bearings.

The alignment bearing keeps the to and bottom half of the sample
running concentrically, while allowing axial motion. By acting on a small
diameter shaft the alignment bearings contribute negligible torque to that
measured by the internal torque cell.

Detail of the sample and the sliding jacket assembly used in
rotary shear. The upper and lower sample rings may either touch each other
directly in the case of experiments on initially bare surfaces, or they
may be separated by a layer of crushed rock that simulates fault gouge.

The jacket assembly consists of O-rings and Teflon rings, against both
the inside and outside boundaries of the sample. The O-ring prevents the
gas that provides the confining pressure from getting to the sample. The
four Teflon rings transmit the pressure from the O-ring to the sample to
provide the confining pressure on the sample. If the O-ring was not
isolated from the sample by the low-friction Teflon, the O-ring would be
torn apart because friction between it and the sample would cause its top
and bottom halves to move in different directions.

Not shown are ports that access the top and bottom sample rings to
allow pressurized pore fluids to flow through them and across the sliding
surface or gouge.

Dr. David Goldsby operating the rotary shear apparatus by entering commands
to the HP computer. The machine itself is in a separate room in order to protect
the operators from harm in case of a catastrophic leak or failure of the
pressure vessel. One of the gas bottles that provides medium pressure gas to the
gas pumping system in the next room is visible to the left, and some of the
pressure gages are visible on the wall and panel. Above the pressure gages on
the panel are shaft handles that run through the wall to the machine and
remotely operate the 1.0 GPa gas valves. Just above those handles are a series
of bubblers connected to plastic tubes that are used to detect the location of
any gas leaks.

From left to right are David Goldsby, Giulio Di Toro, Terry Tullis, and Dr.
Naoyuki Kato, looking at a plot of some experimental data on one of our 19"
color monitors. The data is stored on CDs for ease of later access and because
it is a stable storage medium for long-term data archiving. One of our IBM
RS/6000 computers is on the shelf above the monitor.

Naoyuki Kato adjusting the reference voltage for the axial servo system. In
the rack to the right at the top is a custom-made commercial interface box that
deals with the analog and digital signals to and from the resolver and to the
rotary servo. Below it is an electronics box that we made that controls the upper and lower
pore
pressure systems, including its two servo controllers, and two LVDT signal
conditioners. Below that are the HP 3852 Data Acquisition and Control Interface
units that contain the digital voltmeters, multiplexers, stepping motor
controllers, digital I/O units, HPIB interface to the UNIX computer, etc.

In
the rack to the left from the top is an overload failsafe circuit board, the
signal conditioners for the 3 other LVDTs in the system, servo controllers and
reference voltage generators for the axial and rotary servos. In the gray panels
at the height of Naoyuik's right elbow are manual/computer controls for the gas
booster and intensifier parts of the gas pumping system, for the
electrohydraulic stepping motor and its brake/clutch system for gear changes.
Below that is the HP UNIX computer that controls the machine and data
acquisition.

Giulio Di Toro holding the sample grips into which quartzite samples have
been epoxied. He is sitting in front of the digital plotted that provides areal-time
graphic record of any of our 19 channels of data that we choose to plot to keep
track of the progress of an experiment.

Terry standing on the platform at the level of the pressure
vessel. The pressure vessel is encased with a blue-painted water-cooling
jacket. The view of the pressure vessel is obscured by half of the
flow-through pore pressure system. In the upper part of the view are two
of the four remotely-air-operated valves that allow flow of pore fluid
through the sample during sliding, changing its chemistry, etc. The
half-inch diameter high-pressure piston generating the pore pressure is
seen entering the bottom of the pore pressure intensifier just in front of
the left part of the pressure vessel. The servo valve for the hydraulic
cylinder loading the high-pressure piston is at the bottom edge of the
photo, as is a solenoid valve that is part of a failsafe system that
prevents the pore pressure from getting too high.